1. Field of the Invention.
The present invention relates to semiconductor devices and methods of fabrication, and, more particularly, to selective epitaxy of semiconductor materials such as gallium arsenide and the resultant devices such as heterojunction bipolar transistors and lasers.
2. Description of the Related Art.
Gallium arsenide (GaAs) has emerged as a leading semiconductor material for high speed integrated circuits because of its high electron mobility and its availability in semi-insulating form. Further, the possibility of epitaxial growth of Al.sub.x Ga.sub.1-x As on GaAs and vice versa allows fabrication of structures such as heterojunction bipolar transistors, high electron mobility transistors (HEMTs), quantum well lasers, and superlattices. Epitaxial growth of GaAs (and other III-V and II-VI compounds) can be performed by liquid phase epitaxy, molecular beam epitaxy (MBE), and metalorganic chemical vapor deposition (MOCVD). MBE and MOCVD both provide the ability to grow extemely abrupt p-n junctions and heterojunctions of lattice-matched materials; and such abrupt junctions are required for fabrication of superlattices and quantum well structures.
One of the disadvantages of MBE and MOCVD is the inability to deposit epitaxial material selectively on a substrate rather than as a uniform sheet covering the substrate. Several masks to achieve selective deposition of GaAs on GaAs and Al.sub.x Ga.sub.1-x As have been proposed: T. Lee et al, 29 Appl. Phys. Lett. 164 (1976), used silicon dioxide; S. Hiyamigu et al, 127 J. Electrochemical Soc. 1562 (1980), tried GaAsO.sub.x ; P. Favenner et al, 18 Elec. Lett. 933 (1982), formed the mask by lattice-damaging ion implantation; and J. Harbison et al, B3 J. Vac. Sci. Tech. 743 (1985), invoked tungsten. However, these masks are either insulators or unable to dope the substrate or both. Further, polycrystalline GaAs forms on the masks abutting the epitaxial GaAs growing from the exposed substrate, and this polycrystalline GaAs is difficult to remove and can also act as a short circuit path.
The use of selective growth masks is related to the problem of the base contact in heterojunction bipolar transistors. In particular, for an emitter-up configuration, control of the base doping and width is not easily reconcilable with the collector contact: if a base layer is nonselectively grown by MBE, then contact to the base may be by etching down to the base layer (see Rezazadeh et al, B4 J. Vac. Sci. Tech. 773 (1986)) or by implanting a doped region down to the base layer, but the collector contact must be etched down through the base layer; if a base is formed by localized ion implant (so collector contact is simple), then control of base width and doping becomes a problem, and base contact again may be by etching down (see J. Tully, 7 IEEE Elec. Dev. Lett. 203 (1986)) or implanting a doped region down to the base (see J. Yuan et al, 16 Elec. Lett. 637 (1980) and W. McLevige et al 3 IEEE Elec. Dev. Lett. 43 (1982)).
Similarly, for an emitter-down (common emitter) configuration, device isolation may be by a lattice-damaging implant to fragment a nonselectively grown base layer or by a mesa etch through the base layer. In either case recombination problems are associated with the damaged interface. Further, photolithographic limitations arise for the alignment of doped regions contacting the base and the collector; and self-aligned methods rely on complex lift-off schemes which have inherent reproducibility problems (see Ohshima et al, 1985 IEEE GaAs IC Symp. p. 53 and Chang et al, 7 IEEE Elec. Dev. Lett. 8-10 (1986)).
A problem with heterojunction laser fabrication is the formation of the cavity mirrors. Several methods have been tried, including wet chemical etching by M. Wada et al, 21 IEEE J. Quant. Elec. 658 (9185), reactive ion etching by L. Coldren et al, 37 Appl. Phys. Lett. 681 (1980), and microcleaving by H. Nobuhara et al, 21 Elec. Lett. 718 (1985). These methods have in common the removal of material and the associated surface damage or yield problems. Microcleaving usually gives the best results because of the smooth vertical nature of the cleaved facets; however, microcleaving does not lead to high yield laser/transistor integration on a single chip.
Thus the problems of GaAs and related semiconductor fabrication and devices include the lack of growth-selective masks, lack of conducting plus dopant source growth masks, and lack of devices with selectively grown semiconductor regions.